The present invention is directed to holographic scanning apparatus and more particularly to an optical filter for producing a light beam of uniform intensity.
A hologram is the name given to a photographic film having an interference pattern recorded thereon. The interference pattern is usually produced by a source of coherent light which is normally split into two beams, one of which is directed onto the sheet of photographic film, and the other of which is directed onto a subject. The beam directed onto the subject is scattered onto the sheet of film with waves of varied orientation and wave front curvature. The scattered light waves and the undisturbed beam are reunited at the film where they interfere as patterns of light and dark areas which are photographically recorded.
When a source of coherent light is directed through the patterned film sheet, it is diffracted into a number of diffraction orders. At least one of these orders duplicates the waves that were originally reflected from the subject, and a person viewing the film sees the subject of the hologram in three dimensions, with the subject appearing behind the hologram at a distance equal to the original subject to film distance. If the viewer changes his position and views the hologram from a different angle, the three-dimensional perspective changes and the viewer may thus look around an object in the foreground to see an object behind it.
Holograms have been utilized in scanning an object such as a UPC coded label to read the data contained therein. An example of such a scanning arrangement may be found in U.S. Pat. No. 4,224,509. In manufacturing the holograms used in the above cited scanning apparatus, a laser is used as a source of coherent light. Where it is required that coherent light be of uniform illumination, objective lenses are used to expand the beam and then limit the expansion with a collimating lens since the laser projects a beam whose cross-sectional profile is approximately Gaussian. In order to overcome this deficiency in a laser beam, optical filters have been proposed. An example of such a filter may be found in the U.S. Pat. No. 3,465,347 which consists essentially of a glass substrate with a spherical surface having a layer of light absorbing material. The cost of such a filter is quite expensive. It is therefore the principal object of this invention to provide a method of producing a low cost optical filter which will produce a beam from the laser having uniform intensity.